Gyroscopes (a.k.a., gyros) are devices used to sense rotation. They have become a nearly indispensable tool for use in inertial navigation of objects and vehicles (particularly when reliance on a magnetic compass is insufficient or not possible), as well as many other applications. Rotational sensing is critical for precise guidance, and is often used in conjunction with accelerometers to enable “dead reckoning” of the position, orientation, speed, etc., of a moving object, such as a ship, submarine, satellite, guided missile, and the like. Increasingly, gyroscopes are making their way into non-military applications as well, such as land surveying, self-driving cars, sophisticated, multi-dimensional crash-detection and air-bag deployment systems, game controllers and cellphones.
Common gyroscope configurations include mechanical gyroscopes, MEMS-based gyroscopes, fiber-optic gyroscopes (FOG), ring laser gyroscopes (RLG), other optical waveguide gyroscopes, and a variety of other configurations. A mechanical gyroscope includes a mechanical system having a spinning wheel or disc whose axis of rotation is free to assume any orientation relative to the system. While the wheel rotates, the orientation of its axis of rotation is unaffected by any tilt or rotation of the overall system due to the conservation of angular momentum. By combining multiple spinning wheels whose axes are orthogonal, two- and three-axis systems can be formed. Unfortunately mechanical gyros are typically quite large and massive. They are also typically sensitive to shock and vibration, and can have poor reliability due to fabrication tolerances, as well as mechanical wear over time.
The RLG was developed, in part, to mitigate some of the disadvantages of the mechanical gyro. A RLG senses changes in orientation by exploiting the Sagnac effect, wherein the frequencies of light signals travelling in opposite directions (i.e., clockwise “CW” and counterclockwise “CCW”) within a planar waveguide loop are based on the angular velocity of the waveguide loop within its plane. When the RLG is not rotating (i.e., it is in its quiescent state), the two light signals have the same phase and frequency and the RLG returns a rotation reading of zero. When the RLG is subject to a rotational component within its plane, however, frequency splitting occurs between the CW and CCW light signals, giving rise to a difference in their frequencies that is proportional to the rate of rotation. The RLG then provides an output signal that is based on this frequency difference.
RLGs are particularly attractive in many applications due to their ability to provide extremely precise rotational rate information, lack of cross-axis sensitivity to outside influences, such as shock, vibration, etc., and their high reliability due to a lack of moving parts.
Unfortunately, prior-art fiber optic gyroscopes suffer from an effect often referred to as “frequency lock-up,” which makes them unable to sense rotation that is below a certain threshold. Frequency lock-up (a.k.a., frequency lock-in) occurs when the rate of rotation of the waveguide loop is very low, which yields an optical-frequency difference between counter-propagating optical signals that is very small. Under these conditions, even very small reflections couple the CW and CCW propagating optical beams, causing them to lock together to the same optical frequency, giving rise to a rotation reading of zero. This frequency lock-up is sometimes solved by introducing mechanical dither to the system, but mechanical nature of dither system can reduce the reliability and performance under large shock and vibration.
The need for a fiber-optic gyroscope that does not exhibit frequency lock-up at low rotation rates remains, as yet, unmet in the prior art.